Optical disk drives in current use employ grooved disks and split photodetectors to provide position sensing information. The grooves introduce noise into the data channel, reducing data capacity. Due to thermal and other effects, the area of the spot imaged on the photodetector may not be equally split between the two detector halves. Also, grooved disks do not contain track-type information.
To overcome these and other problems, sector servo systems have been proposed with circumferentially spaced sector patterns comprising "features" (embossments or depressions) that extend radially from the inner to the outer diameter of the data band. The data recording surface of the data band is flat between the sector pattern so there is no signal degradation. The servo pattern edges (and hence diffraction orders) used for tracking error signals (TES) are separated in time, directed onto a single (rather than split) photodetector, and used to calculate the TES. Offsets due to tilt, objective lens translation, and thermal shift are eliminated because there is no split detector. Finally, there is no coupling between the tracking features and focus error signal (FES) because the laser beam can be focused on the flat data recording area and turned off when over the sector area containing position information.
A consortium of Japanese manufacturers is proposing for low end optical drives a sector servo system in which the position of "di-bits" is varied to encode track type information. It uses di-bits in the form of dots, thereby providing a track error signal of very limited linearity. Also, it provides no means to compensate for radial velocity error. During radial seek operations, spots are sensed as being closer; as a result of this distortion of the velocity in relation to the change in radial distance, the radial distance is not accurately sensed. Finally, it provides no means to automatically compensate for variations in pattern size that may occur during fabrication of the disk.
In the commonly owned copending application, U.S. Ser. No. 07/392,035, filed Aug. 8, 1989, now U.S. Pat. No. 5,134,601, issued Jul. 28, 1992, a sector servo system is disclosed which employs a multi-phase servo pattern that provides both fine tracking and coarse seeking position information. The fine tracking pattern provides, for each phase, sets of uniformly spaced edges whose positions are fixed by the track pitch and cannot be varied. While this system works satisfactorily, the track error signal has limited linearity and, due to the nonvariable relationship of the edges, it is not possible to optimize the edge spacing to minimize sensitivity to tilt and to variations in spot size.
No prior art known to applicant provides a sector servo system having two interleaved sets of servo patterns, those of one set being a permutation (i.e., a reversal and/or the complement) of those of the other set, thereby to provide a tracking error signal which automatically compensates for variations in radial velocity of an optical head as it moves generally radially of the disk and/or cancels the effects of process variations during fabrication of the disk. Nor does any known prior art provide a sector servo pattern having for each phase two edges that are independently variable.